Lithium ion secondary batteries are lightweight and have a high energy density, and therefore use of lithium ion secondary batteries as a power source for small electronic devices, automobiles, or houses has been examined. Upon production of lithium ion secondary battery electrodes, usually, a polymer binder is used as the binder, and an active material (positive electrode active material and negative electrode component material) is blended with the polymer binder to prepare an electrode composition, and the electrode composition is applied on the current collector and dried, thereby binding the active material to the current collector. The polymer binder is required to be adhesive to the active material, adhesive to the current collector, resistant to a polar solvent which is the liquid electrolyte, and stable under an electrochemical environment.
Conventionally, a fluorine-based polymer such as polyfluorovinylidene has been used as such a polymer binder. However, the fluorine-based polymer has to be dissolved in an organic solvent, and there is a disadvantage in that when the electrode composition is applied to the current collector and then dried, the organic solvent is volatilized. Also, there is a disadvantage in that because of its poor binding capacity, the polymer binder has to be blended in a large amount to obtain sufficient binding capacity, which inhibits conductivity of the secondary battery.
Thus, to make an improvement in the above-described disadvantages, various proposals have been made to use an aqueous dispersion of a non fluorine-based polymer as the polymer binder.
For example, Japanese Unexamined Patent Publication No. H5-74461 (Patent Document 1) has proposed using a styrene-butadiene latex having specific composition and gel content as the polymer binder to obtain a secondary battery with excellent cycle characteristics, storage characteristics, and safety.
Furthermore, Japanese Unexamined Patent Publication No. H11-25989 (Patent Document 2) has proposed using a water-based dispersion of a copolymer having specific composition and glass transition temperature as the polymer binder to obtain a secondary battery with high capacity, excellent discharge performance, charge and discharge cycle characteristics, and safety.
Furthermore, Japanese Unexamined Patent Publication No. H8-250122 (Patent Document 3) has proposed using a styrene-butadiene latex having a butadiene content of a specific range as the binder, and drying the binder at 50° C. or more to obtain a battery electrode with excellent cycle characteristics, storage characteristics, and safety.